1. Field of the Invention
The present invention relates to an integrated scanning probe microscope sensor (hereinafter referred to as SPM sensor) for use in a scanning probe microscope.
2. Description of the Related Art
Optical microscopes, which achieve a high resolving power in excess of diffraction limited to one using an evanescent wave, have been proposed since the 1980s. Such a microscope is called a scanning near field optical microscope (hereinafter referred to as SNOM). The SNOM is classified as an SPM, as are the scanning tunneling microscope (hereinafter referred to as STM) and the atomic force microscope (hereinafter referred to as AFM). The SNOM makes use of the phenomenon that the evanescent wave is confined to a localized region which is less than its wavelength and is unable to propagate in free space.
The principle of measurement in SNOM is as follows: a probe of the microscope is placed in a close vicinity of the surface of a sample to be measure by a distance of no more than one wavelength; and the intensity of light that passes by the miniature slit at the tip of the probe is mapped to form an image of the sample. Although several methods to implement SNOM have been proposed, they are divided into two main methods. One is called a collection method, in which light is directed from below the sample, and to form an SNOM image, a probe is used to pick up the evanescent wave that is transmitted through the sample and localized in the vicinity of the surface of the sample. The other is called an emission method, in which a probe having a miniature slit projects light to a sample, and the light transmitted through the sample is picked up by a photodetector device mounted below the sample. This method has been disclosed, for example, in Japanese Patent Application Laid-open No. Hei-4-291310 (AT&T; R. E. Betzig).
A typical SNOM apparatus is now discussed. A sample to be measured is placed on top of an inverted prism on a three-dimensionally movable stage. A semiconductor generated laser beam irradiates the sample bearing surface of the prism at an angle that satisfies total reflection conditions. As a result, evanescent light is generated in the vicinity of the surface of the sample. When a fiber-optic probe having a sharpened tip is placed close to the surface of the sample to be measured, evanescent light is converted into scattering light. Scattering light is guided via the fiber-optic probe to a photodetector device, which then detects a change in the intensity of scattering light. The change in the intensity of scattering light detected by the photodetector device is converted to corresponding scattering light intensity signal, which is then output to a Z position control mechanism. In response to the scattering light intensity signal, the Z position control mechanism controls the three-dimensionally movable stage to move it in the Z direction, and holds the sample and the tip of the fiber-optic probe in approximately the same position.
In the above test setup, a microcomputer controls the three-dimensionally moving state to move it in the XY plane according to the XY scanning mechanism. The fiber-optic probe thus scans relatively the sample in XY plane. Evanescent light generated in the vicinity of the surface of the sample is picked up the fiber-optic probe, which in turn converts evanescent light into scattering light. Scattering light is converted by the photodetector device into an electrical signal that represents the intensity of light. The electrical signal is subjected to image processing such as noise elimination and background cutting, and then is displayed as an SNOM image.
The design of such an SNOM probing apparatus is discussed referring to FIGS. 1A and 1B. The probing apparatus in FIGS. 1A and 1B has been presented in the Papers for the 55th Conference of the Applied Physics Society of Japan (Vol. 2, P457, autumn of 1994). FIGS. 1A and 1B are plan and cross-sectional views showing the probing apparatus. In FIGS. 1A and 1B, a cantilever portion 101, made up of a supporting base 102 and a lever 103, is constructed of a p-type silicon substrate. Also shown are an n-type diffusion layer 104 formed on the tip of the lever 103 that is constructed of the p-type silicon substrate, an n+ diffusion layer 105 for establishing an ohmic contact with a signal line, a p+ diffusion layer 106 for establishing an ohmic contact with a signal line, the signal line 107 connected to the n+ diffusion layer 105, and the signal line 108 connected to the p+ diffusion layer 106. Designated 109 is a photodiode region constructed of the n-type diffusion layer 104 and the p-type silicon substrate that forms the lever 103.
The operation of the SNOM probe thus constructed is now discussed. The signal line 108 is supplied with a negative voltage relative to the signal line 107. This negative voltage reverse biases the junction between the n-type diffusion layer 104 in the photodiode region 109 and the p-type silicon substrate forming the lever 103. A depletion layer is thus generated in and near the junction, and thus the junction is put into a depletion state. When incident light such as evanescent light enters the photodiode region 109 in this state, hole-electron pairs are generated in and near the depletion layer. The optical-signal signal current resulting from the generation is then picked up via the signal lines 107, 108, and the sensing of the optical signal is thus possible.
As a microscope that allows one to observe with the accuracy to the size of atoms a dielectric sample that typically presents measurement difficulties with STM, AFM has been proposed (Japanese Patent Application Laid-open No. Sho-62-130302: IBM, G. Binnig, method and apparatus for image forming of the surface of a sample).
The design of this AFM is similar to that of STM, and thus the AFM is categorized as a type of SPM. In AFM, a cantilever having on its free end a sharp tip (probe) is placed close to a sample, and the motion of the cantilever that is displaced by an interactive force working between the atoms of the probe tip and the atoms of the sample is electrically or optically measured. While the sample is scanned in the XY plane, the irregularity of the sample is thus three-dimensionally captured by allowing the probe tip of the cantilever to move relative to the sample.
In this AFM, the displacement measuring sensor for measuring the displacement of the cantilever is typically separately devised. Recently, however, M. Tortonese et al. have proposed an integrated AFM sensor in which the function of displacement measurement is implemented into the structure of a cantilever. Such an integrated AFM sensor is disclosed, for example, by M. Tortonese, H. Yamada., R. C. Barrett and C. F. Quate in a paper entitled "Atomic force microscopy using a piezoresistive cantilever" (Transducers and Sensors '91) and in PCT Patent Application WO92/12398.
In the SNOM probing apparatus as shown in FIGS. 1A and 1B, the cantilever 101 is constructed of the p-type silicon substrate. The area where the p-type substrate is exposed, for example, the exposed surface 110 of FIG. 1B is shown in an enlarged view in FIG. 2. In FIG. 2, the exposed surface 110 of the p-type silicon substrate has a natural oxide layer (SiO.sub.2 layer) 111 having a thickness ranging from a several .ANG. to tens of .ANG.. Positive interface charge 113 at a density of 10.sup.10 to 10.sup.12 cm.sup.-2 exists in the natural oxide layer 111 or Si--SiO.sub.2 interface 112. The presence of the positive interface charge 113 induces an electron inversion layer 114 or an acceptor depletion layer 115 on the surface of the lever 103 of the p-type silicon substrate.
In the above state, hole-electron pairs are generated by thermal excitation in the interfacial level existing in the Si--SiO.sub.2 interface 112, or in the acceptor depletion layer 115 in which the generation-recombination center exists. The hole-electron pairs are noise detrimental to optical signal. For example, holes generated in the exposed layer 110 flow to the signal line 108 via the p+ diffusion layer 106. Electrons flow to the signal line 107 via the n+ diffusion layer 105. They are superimposed on the optical signal as noise. As seen from FIGS. 1A and 1B, in the known SNOM probing apparatus, the cantilever portion 101 is constructed of p-type silicon substrate, and the majority of the surface of the cantilever 101 work as a dark current generation region. A great deal of dark current is thus generated, severely degrading the S/N ratio in the signal detected by the photodiode.
The integrated AFM sensor disclosed by M. Tortonese et al. is a cantilever into which a strain sensor is integrated. A cantilever integrated with sensor having photodetecting capability may be easily contemplated. If a sensor having photodetecting capability, such as a photodiode, is simply integrated into a cantilever, however, photo-carriers arising from exposure to light irradiation may recombine with carriers in the semiconductor, or may be captured by traps in the semiconductor. Photo-carriers therefore fail to contribute as photocurrents, lowering sensing efficiency. A feeble light, such as evanescent light, exists only in the vicinity of the surface of the sample and is unable to reach the depletion layer. Photo-carriers generated will immediately recombine in the semiconductor, and the sensitivity of the sensor is thus lowered.
In an SNOM apparatus having a known fiber-optic probe, a separate sensor is mounted external to the probe, causing its design to be bulky. Such a bulky design not only exposes the SNOM apparatus to ambient vibrations and shocks, but also presents manufacturing difficulty. Since the probe is separated from the photodetecting mechanism, light suffers a loss therebetween. Photodetecting efficiency, namely, sensitivity is thus degraded. The prior art SNOM apparatus is totally different in system configuration from SPM apparatuses and AFM apparatuses in particular. Therefore, many users are obliged to purchase a dedicated SNOM apparatus besides an AFM apparatus. This means a substantial increase in expenditure to the users.